miércoles, 3 de abril de 2013

While a high-pressure weather system
brought warmer than normal temperatures to Greenland and northern Canada
in March 2013, much of North America, Europe, and Asia shivered through
weeks of unseasonably cool temperatures. The contrasting temperatures
are no coincidence: the same unusual pressure pattern in the upper
atmosphere caused both events.Atmospheric pressure patterns are constantly in flux, as air masses
of differing temperatures and densities move around the skies. One key
measure of pressure that meteorologists track closely is known as the Arctic Oscillation (AO) index,
the difference in relative pressure between the Arctic and the
mid-latitudes. Changes in the AO have can major impacts on weather
patterns around the world.When the AO index is in its “positive” phase, air pressure over the
Arctic is low, pressure over the mid-latitudes is high, and prevailing
winds confine extremely cold air to the Arctic. But when the AO is in
its ”negative“ phase, the pressure gradient weakens. The pressure over
the Arctic is not as low and pressure at mid-latitudes is not as high.
In this negative phase, the AO enables Arctic air to flow to the south
and warm air to move north. In late March, the AO dropped as low as -5.6. (See this chart published by the Washington Post’sCapital Weather Gang to see how this compares with other periods when AO values reached record-low levels).The temperature anomaly map above, based on data from the Moderate Resolution Imaging Spectroradiometer (MODIS) on NASA’s Aqua
satellite, shows how this affected temperatures in the Northern
Hemisphere. The map displays land surface temperature anomalies between
March 14–20, 2013, compared to the same dates from 2005 to 2012. Areas
with above-average temperatures appear in red and orange, and areas with
below-average temperatures appear in shades of blue. Much of Europe,
Russia, and the eastern United States saw unusually cool temperatures,
while Greenland and Nunavut Territory were surprisingly warm for the
time of year.Many parts of the Northern Hemisphere saw near record-breaking cool
temperatures as the value of the AO fell. The United Kingdom experienced
its 4th coldest March since 1962. In late-March, two-thirds of weather stations in the Czech Republic broke records. Germany saw its coldest March since 1883. And Moscow had its coldest March since the 1950s.

A dust storm blew out of Libya and
across the Mediterranean Sea in late March 2013. Southwest of the
coastal city of Banghazi (Benghazi), an especially thick dust plume
spanned roughly 100 kilometers (60 miles), and the plume was thick
enough to completely hide the ocean surface below.The Moderate Resolution Imaging Spectroradiometer (MODIS) on NASA’s Terra
satellite acquired this natural-color image of the dust storm on March
30, 2013. The dust plumes arose hundreds of kilometers inland, and dust
stretched across the Mediterranean Sea toward southern Italy. (The
extent of the dust storm can be seen in the high-resolution image.)Dust storms count among the most frequent natural hazards in Libya. A
relatively mild Mediterranean climate predominates along the coast, but
vast sand seas sprawl over the interior landscape. Those sand seas
provide plentiful material for dust storms, and hot, dry, dusty winds
can last up to four days in the spring and fall. In this country where
water is so scarce, development projects pull water from aquifers to be used for irrigation.